Add a changelog entry

2019-11-13 12:00:26 +01:00 · 2019-11-13 12:00:26 +01:00 · b34749c1fb
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 [Full changelog](https://github.com/mozilla/glean/compare/v20.1.0...master)

+* The `GleanTimerId` can now be accessed in Java and is no longer a `typealias`.
+
 # v20.2.0 (2019-11-11)

 [Full changelog](https://github.com/mozilla/glean/compare/v20.0.0...v20.1.0)